120 



ZOOLOGY 



SECT. 



precisely the same layers as that of the hydranth ectoderm,, 

 mesogloea, and endoderm. The ectoderm is continued on to the 

 sub-umbrella, and then round the margin of the bell on to the 

 ex-umbrella, so that both surfaces of the bell are covered with 

 ectoderm. The endoderm is continued from the base of the 

 enteric cavity into the radial canals, and so to the circular canal, 

 so that the whole canal-system is lined by endoderm. In the 

 portions of the bell between the radial canals there is found, 

 between the outer and inner layers of ectoderm, a thin sheet of 

 endoderm, the cndodcrm-lamclla (end. lam), which stretches 

 between adjacent radial canals and between the circular canal 

 and the enteric cavity. In the bell, as in the manubrium, a 



errfi ' /nrn 



i 



FIG. 88. Dissection of a medusa with rather more than one-quarter of the umbrella and manu- 

 brium cut away (diagrammatic). The ectoderm is dotted, the endoderm striated, and the 

 inesoglcea black ; circ. c. circular canal ; mil. lam. endoderm lamella ; yon. gonad ; /. lithocyst ; 

 in nil. manubrium; mth. mouth; rod. c. radial canal; -cl. velum. 



layer of mesogloea everywhere intervenes between ectoderm and 

 endoderm. 



The velum (vl) consists of a double layer of ectoderm and a 

 middle one of mesogloea : there is no extension of endoderm into 

 it. The tentacles, like those of the hydranth, are formed of a 

 core of endoderm covered by ectoderm, the cells of the latter 

 being abundantly supplied with stinging-capsules. 



Comparison of Polype and Medusa. Striking as is the 

 difference between a polype and a medusa, they are strictly 

 homologous structures, and the more complex medusa is readily 

 derivable from the simpler polype form. It is obvious, in the 

 first instance, that the apex of the umbrella corresponds with the 

 base of a hydranth (Fig. 89, A and D), being the part by which 

 the zooid is attached in each case to the parent stem : the mouth 

 and the manubrium are also obviously homologous structures in. 



